A polypropylene resin has conventionally been used in various fields, because of being relatively cheap and having many superior characteristics.
However, because of limited applications as a simple substance of polypropylene, various improvements have been added. For example, there have been proposed a propylene-based block copolymer obtained by homopolymerization of propylene and then copolymerization of propylene and ethylene, to enhance impact resistance, or a propylene-based polymer with widened molecular weight distribution obtained by stepwise production of polypropylene with different molecular weight, to improve formability and appearance.
In producing these resins, there is a method for kneading again each component in predetermined ratio, after producing each component separately, however, from economical efficiency and worry of decrease in quality by kneading, a multi-stage continuous polymerization method for producing each component continuously is advantageous. However, because polymer particles become nearly a completely mixed state in one container, short pass where not sufficiently grown particles are exhausted, or accumulation of too grown particles inside the container easily occurs, thus causing decrease in quality of said polymer. To solve this, it is also considered to attain plug flow as a whole by connecting a plurality of continuous stirred tank reactors in series, however, installment of many reactors requires equipment cost.
In one reactor, there has been known, as a known one for attaining plug flow by decreasing residence time distribution, a horizontal-type reactor having a stirring machine which rotates around a horizontal axis (a horizontal-type vapor phase method process, for example, refer to PATENT LITERATURE 1. Hereafter a reactor may be referred to as a reaction tank.). In general, catalyst particles gradually grow to polymer particles by a polymerization reaction, however, in the case of performing polymerization in the horizontal-type reactor, these particles proceed along an axis direction of a reactor while gradually growing, by two forces of generation of polypropylene by polymerization and mechanical stirring. Therefore, particles with the same degree of growth, that is, residence time align with time from the upstream toward the downstream of the reactor. That is, in the horizontal-type reactor, a flow pattern of fluid becomes a plug flow-type, which has effect of narrowing residence time distribution in the same degree as in the case of aligning a plurality units of continuous stirred tank reactors in series. Accordingly, in producing said polymer, the propylene polymerization reaction apparatus composed by connecting multiple units of horizontal-type reactors in series is advantageous.
It should be noted that, in the case of aiming at improvement of polypropylene shown by the above-described example, a component having low molecular weight, or a component having many ethylene components or the like is produced in a certain polymerization step. In the horizontal-type reactor, latent heat of liquefied propylene is utilized to remove heat of polymerization. As liquefied propylene, the liquefied one obtained by taking out unreacted propylene gas from a polymerization reaction tank and then by cooling with a heat exchanger, may be used in some cases. Because temperature (dew point) where unreacted gas is liquefied depends on pressure and a composition of unreacted gas, a large quantity of mixing of a gas component having lower dew point, such as hydrogen or ethylene, into propylene, as compared with dew point of single propylene, decreases dew point accompanying with increase in mixing amount. As a result, in a process including the horizontal-type reactor, a problem of significant decrease in productivity in view of heat removal is raised, in producing a multi-stage polymer containing a component having low molecular weight, or a component such as ethylene.
To solve the above problems, there is also a method for enhancing cooling capability of a heat exchanger, however, that case requires tremendous equipment cost. Provided that the heat exchanger having high capability is installed, its operation requires tremendous energy.
In this way, although a horizontal-type vapor phase method process has superior plug flow characteristics, the case of producing a multi-stage polymer having a component containing low molecular weight, or a component having ethylene or the like in high content has a problem to be solved in view of productivity and operation cost.
In addition, crystalline polypropylene is cheap and has superior characteristics in rigidity and heat resistance, however, had a problem that impact strength, in particular, impact strength at low temperature is weak. As a method for improving this point, there has been known a method for forming a block copolymer by step-wise polymerization of propylene and an α-olefin or other olefin. A propylene-based block copolymer is composed of a crystalline propylene polymer part and a amorphous propylene/α-olefin copolymer part, and is capable of preparing a molded article having high rigidity and superior impact resistance at low temperature. Accordingly, it has widely been used in applications of automotive parts, electric appliance parts and the like.
It should be noted that in molded articles made of polyolefin to be used in automobile and electric appliance applications, there are applications requiring gloss, while there are also applications requiring low gloss, because molded articles with low gloss provide feeling of luxury. In particular, in applications of automotive interior or exterior materials, a material showing low gloss is desired.
In the propylene-based block copolymer, furnishing of impact resistance is mainly attained by increasing content of the amorphous propylene/α-olefin copolymer, as well as suppression of gloss is attained by increasing molecular weight of the amorphous propylene/α-olefin copolymer and content of the α-olefin. However, these methods not only increase gel in the propylene-based block copolymer, and deteriorate appearance of a product caused by a large quantity of gel but also cause to decrease impact resistance. To begin with, because main cause of gel generation is generation of distribution of polymerization time (residence time in a polymerization tank) of a catalyst component in the first stage polymerization step, and generation of particles having high content of the amorphous propylene/α-olefin copolymer, when particles exhausted from the polymerization tank in relatively short time (bypassing particles) enter a polymerization tank of the second stage polymerization step, in the multi-stage continuous vapor phase polymerization method, it can be reduced significantly by adopting a batch method. However, a batch method has a problem of being inferior in view of economical efficiency and productivity.
As a method for producing a propylene-based block copolymer having low gloss by the multi-stage continuous vapor phase polymerization method, there has been disclosed a method for increasing molecular weight of the amorphous propylene/α-olefin copolymer or content of the α-olefin (for example, refer to PATENT LITERATURE 2 and 3), however, because the first stage polymerization step is performed in the continuous stirred tank reactor, there is left a problem in view of suppression of gel. On the other hand, as a polymerization method in which gel is suppressed, there has been known a method for using a horizontal-type reactor (for example, refer to PATENT LITERATURE 1). This method is advantageous in view of suppression of gel, because of narrow residence time distribution in a reaction tank, which enables to reduce bypassing particles in one reaction tank, however, has a problem of generation of composition distribution and easy providing high gloss in producing the amorphous propylene/α-olefin copolymer, because of removing heat of polymerization by latent heat of vaporization of liquid propylene, causing a non-uniform gas composition in a reactor. As another method, there has been disclosed a method for performing the first stage polymerization step by connecting three reaction tanks in series, and subsequently producing a copolymer component having increased content of the α-olefin (for example, refer to PATENT LITERATURE 4). This method improves as for suppression of gel, however, requires installment of many reactors, as well as has a problem of inability of producing desired amount of a copolymer in the second stage polymerization step, because of longer residence time in the first stage polymerization step, which decreases capability of a catalyst.
On the other hand, it has been described in the above-described PATENT LITERATURE 2 that molecular weight of the amorphous propylene/α-olefin copolymer is also an important factor in producing said propylene-based block copolymer. In the multi-stage continuous vapor phase polymerization method, because transfer of a polymer between polymerization reactors is performed by pressurized transfer, gas in a vapor phase polymerization reactor of the first stage polymerization step is also sent to the second stage polymerization step entrained with the polymer, therefore a gas composition in the vapor phase polymerization reactor of the first stage polymerization step is largely influenced by that of the first stage polymerization step. For example, to obtain a polymer having desired molecular weight, generally hydrogen is used in many cases as a molecular weight modifier, however, in the case of supplying a large quantity of hydrogen to obtain a polymer with low molecular weight in the first stage polymerization step, even trying to obtain a copolymer with high molecular weight without supplying hydrogen in the second stage polymerization step, hydrogen in entrained gas from a reactor of the first stage polymerization step inevitably flows into a reactor of the second stage polymerization step, resulting in generation of limitation in molecular weight of a copolymer formed in the second stage polymerization step.
As a method for solving this problem, there has been disclosed a method for diluting entrained gas with inert gas, in a receiver installed between polymerization reactors, to lower a hydrogen gas composition, and after that for transferring it to the second stage polymerization step (for example, refer to PATENT LITERATURE 6), however, this method cannot prevent flowing-in of hydrogen or the like completely, and could incur fluctuation of partial pressure of a monomer. As another method, there has been disclosed a method for receiving a polymer into said receiver, and then once exhausting entrained gas in said receiver, and transferring it to the second stage polymerization step by pressurization again using propylene gas or inert gas, (for example, refer to PATENT LITERATURE 7). This is a method for suppressing flow-in of hydrogen or the like to the second stage polymerization step to the utmost, however, because of necessity to supply propylene or inert gas into said receiver, till completion of transfer of a polymer remaining inside the receiver or a pipeline, excess propylene or inert gas flows-in to the second stage polymerization step, which could fluctuate partial pressure of a monomer. As a further another method, there has been disclosed a method for installing a receiver at a position higher than a reactor of the second stage polymerization step, and using circulation gas of the second stage polymerization step as pressurized gas (for example, refer to PATENT LITERATURE 8), however, because of use of circulation gas of the second stage polymerization step containing a comonomer such as ethylene, there is left a problem of risk of forming a sticky polymer in the receiver and inside a pipeline.
In addition, in particular, in automotive members, it has been desired enhancement of balance of rigidity/impact resistance at low temperature of a propylene-based block copolymer, as well as attaining high fluidity of said copolymer, aiming at making larger and lighter members.
Generally, in attaining high fluidity of the propylene-based block copolymer, it is mainly attained by attaining high fluidity (decreasing molecular weight) of the crystalline propylene polymer part. As another method, there is also a method for decreasing molecular weight of an amorphous propylene/α-olefin copolymer component, however, this method decreases at the same time impact resistance at low temperature of the propylene-based block copolymer, and is thus not preferable to attain the object of the present invention.
In general, to adjust molecular weight of the crystalline propylene component, a molecular weight modifier such as hydrogen is used, however, in particular, to express high fluidity of the propylene-based block copolymer the present invention desires, it is necessary to maintain high hydrogen concentration inside a polymerization reactor in the first stage polymerization step, and further maintain low hydrogen concentration in a copolymerization step.
As a method for producing the propylene-based block copolymer with enhanced impact resistance by the multi-stage continuous vapor phase polymerization method, there has been disclosed a method for increasing molecular weight of the amorphous propylene/α-olefin copolymer or content of the α-olefin (for example, refer to PATENT LITERATURE 2 and 3), however, because the first stage polymerization step is performed in the continuous stirred tank reactor, there is left a problem in view of suppression of gel. On the other hand, as a polymerization method in which gel is suppressed, there has been known a method for using a horizontal-type reactor (for example, refer to PATENT LITERATURE 5). This method has a problem that condensation of unreacted gas becomes difficult and content of a comonomer cannot be increased, in the case of using a comonomer having low boiling point such as ethylene, as the α-olefin to be used in the second stage polymerization step, because removal of heat of polymerization is performed by latent heat of vaporization of liquid propylene, although having advantage in view of suppression of gel, because residence time distribution inside a reaction tank is narrow and thus bypassing particles in one reaction tank can be reduced. Further, because of using two horizontal-type reactors, in continuous operation, it is difficult to produce a copolymer component having desired high molecular weight due to leakage of hydrogen from the first reactor.
As another method, there has been disclosed a method for connecting three continuous stirred tank reactors in series, to perform the first stage polymerization step in the first reaction tank, and produce a copolymer component having increased content of the α-olefin in the second and the third reaction tanks (for example, refer to PATENT LITERATURE 4 and 9). The method of PATENT LITERATURE 4 requires setting of restriction of molecular weight of a polymer in each tank for gel suppression, and thus has a problem in more increasing molecular weight of the amorphous propylene/α-olefin copolymer component. In addition, the method of PATENT LITERATURE 9 has a problem in view of increasing productivity while maintaining quality of the propylene-based block copolymer, such as requiring setting of restriction of production speed in the first stage polymerization step.
Further, as another method aiming at suppression of gel in a three-stage polymerization method, or reducing adhesion or the like in the reaction tank, there has been proposed a method for specifying amount of an electron donor compound to be added to the second and the third tanks (for example, refer to PATENT LITERATURE 10). However, this method requires addition of a relatively large quantity of the electron donor compound to the second reactor, which raises restriction of reaction amount of a copolymer production in each reactor, depending on amount the electron donor compound to be added.
Accordingly, in multi-stage continuous vapor phase polymerization method, there has been desired a propylene polymerization reaction apparatus or a production method of a propylene-based polymer, which are capable of producing a high quality continuous multi-stage polymer such as the one having superior balance of rigidity/impact strength, as well as suppressed gel generation, and also suppressed gloss, high fluidity, or the one having a wide composition with suppressed adhesion inside a reaction tan, in low cost and stably, without decreasing productivity, even in the case of polymerization of a component having low molecular weight.